except for when they get a warrant and take a blood sample.
Just because they CAN doesn't mean they will. Had a friend refuse to blow last week and no blood sample was even brought up in conversation.
Also unless its a checkpoint with the big bus, the time it takes to obtain the warrant and take you to the hospital not only takes long enough for your blood alcohol level to lower.. It decreases that cop's chance of catching people who don't know not to blow so he likely won't bother with you.
Blood samples really only come into play when its a DUI setup with someone ready to take blood.